Feather-like structures in a positive streamer discharge

If a sufficiently high voltage is applied to a volume of ionizable matter, a electrical discharge channels its way through in the form of a tree with growing branches. The ionized branches that penetrate the ionized matter are called streamers. Everybody knows the momentary beauty and elegance of an randomly outstretched tree of streamer if a flash appears on the horizon. Using a high speed ICCD camera the evolution of electrical discharge can be studied and the beauty of feather-like structures in a positive streamer discharge can be captured.

Nonlinear nature of electrical discharges

Streamer discharges determine the very first stage of sparks or lightning. They govern in nature the evolution of huge sprite discharges above thunderclouds and in human made technology the operation of corona reactors in plasma technology. The electrical discharge can occur in stunning beauty as it can be seen in Figure 1. Furthermore, this picture shows the fact that the streamers are nonlinear structures with multiple inner scales. The captured discharge propagates through argon at standard temperature and pressure. At the upper middle of the picture, an invisible electrode needle supplies a positive voltage pulse and the photograph integrates the light emission over about a microsecond. Also, the streamer trees are three-dimensional objects and this structure of the electrical discharge can already be recognized as some branches on the picture are out of focus.

Dynamics of electrical discharges

Although, lightning effects in nature seem to happen simultaneous the high shutter speed of an ICCD camera enables an insight to the process of lightning. This sequence of streamers can be seen in Figure 2 as a series of images shows the devolution of positive and negative streamer. The left column (panels a, b and c) shows positive streamer discharge trees which means the invisible needle on the top of the image is positive charged. Compared to the right column which shows negative streamer, hence with a negative charged needle. Panels (b) and (c) show that not the whole channel emits light, but only the pieces of the channel that grow during the exposure time of the camera. Another pedagogical illustration of the effect that only the actively growing tips of the channels emit light. Adjusting the opening and exposure time of the ICCD camera therefore has become an important tool in analyzing streamer dynamics. With exposure times as short as 300ps, even the shell type inner structure of the ionization front within the streamer tip could be reconstructed.